1. Field Of The Invention
The present invention relates to the manufacture of plastic film and of web materials coated with plastic film.
More particularly, the present invention relates to a method of testing the finish ground surface texture of chill rolls for plastic film extrusion machines and to apparatus for performing the test.
2. Description Of The Prior Art
In the manufacture of film products from thermoplastic polymer materials such as polyethylene, polyvinychloride, polypropylene, polyesters, and ethylenevinyl acetates, it is known to extrude the plastic material in a viscous, molten state onto a rotating chill roll.
Similarly, web products such as paper are coated with such plastic films by extruding a molten flow of the material onto a traveling web of paper substrate. Immediately following contact with the paper web, the plastic is cooled by surface contact with a rotating chill roll.
Since the plastic is heated to a viscous, liquid state for extrusion, when the hot material initially contacts a chill roll, the roll surface texture is cast into the surface of the plastic. Consequently, the surface finish of a chill roll is the primary determinator for the surface finish of the final product.
The measurement of surface finish as a machine shop practice for the manufacture of roll elements is normally accomplished by means of an instrument known as a profile measuring device which determines the root mean square deviation of surface profile irregularities from an average surface plane. The disclosure of U.S. Pat. Nos. 3,253,370; 3,613,319; and 3,835,597 are directed at this method of roll finish measurement.
For measurement of paper, plastic and sheet product surface finish, however, the technique of spectral reflectance is used. This technique is defined by ASTM test method D523. Instruments for performing the ASTM test method D523, known as glossmeters, are described in U.S. Pat. Nos. 1,988,556; 2,063,551; 2,471,750; 2,739,246 and 3,549,264.
In practice, there has been little or no correlation found between the two test methods since a given profile measuring device for different surfaces may produce vastly different glossmeter measurements. This lack of correlation has generated considerable consternation with the plastic coated paper and film industries due to the fact that chill roll surfaces are frequently damaged in use. Such an event necessitates the need for roll surfacing refinishing. However, since there is no correlation between profile measuring device and glossmeter measurements, it is often necessary to install a refinished chill roll and make the desired product with it to determine that the finish texture of the product is incorrect. If incorrect, the subject roll must again be removed from the production machine for further finishing.
Analysis of the problem would seem to suggest that a glossmeter instrument should be used to monitor final finishing of a roll on the finish polishing machine. However, as heretofore understood, the operational principals of gloss measurement are predicated on the flatness of the test subject. This criterion is repeated throughout literature on the subject. The cylindrical surface of a chill roll obviously does not conform to the gloss measurement premise. Consequently, heretofore, the glossmeter was not known to the machinist.